1. Background of the Invention
The present invention relates to a method of controlling the carbon content in a molded metallic piece and to a method of making sintered metallic pieces from a metallic carbon containing powder.
2. Description of the Background
Control of carbon content is one of the principal issues related to injection molding of metals. The difficulty arises mainly from the binder used for shaping, which decomposes during heat treatment and results in carburization. Residual carbon can be beneficial for materials, such as carbides, but there are instances where an excess of carbon is detrimental; for example, stainless steel, magnetic alloys and steels for which the carbon content must be carefully adjusted. The variation in carbon content may be due to a carburization arising from an incomplete binder degradation, but also from reaction in situ between carbon and the oxygen impurities of the powder or between carbon and the oxygen, or vapor water impurities of the furnace atmosphere. The effect of the atmosphere content on the powder during sintering is disclosed by D. R. Ryan and L. J. Cuddy in "Effect of Atmosphere Composition on the Sintering Behavior of Iron Powder Compacts", Pennsylvania State University.
The carbon content of the parts can be adjusted during a specific step after debinding, before sintering. The gas used for the treatment is usually a mixture of carbon monoxide and carbon dioxide. The carbon content of the compacts, c, is adjusted via the carbon potential of the atmosphere, ac, fixed by the CO/CO.sub.2 ratio, according to the following relation: EQU c (wt. %)=ac with ac&lt;1
It is necessary that the treatment be done in a temperature range where the porosity of the sample is still high (about 20%) to allow rapid equilibrium between carbon content of the parts, throughout all the thickness, and carbon potential of the gas.
In practice, the control of carbon content via the mixtures CO--CO.sub.2 is difficult to achieve because the CO/CO.sub.2 ratio as well as the temperature of the treatment must be adjusted with precision. In an industrial furnace, temperature gradients and oxygen impurities in the flowing gas can shift the CO/CO.sub.2 ratio and modify the carbon content of the parts. Moreover, this type of treatment is done in a batch furnace and is costly because it is difficult to automate and, is, moreover, time consuming.
Ideally, the carbon content of the compacts would be controlled throughout the debinding, wherein the binders are removed. Thus, the debinding should ideally and desirably afford a complete and clean decomposition of the binders and a reduction of the oxides of the powder. Preferably, the dew point and the oxygen impurities of the flowing gas during debinding and sintering could be lowered to a level where they would not influence the carbon content. In such a cased after sintering, the carbon content of the parts would be that of the starting powder.
However, at present, such a process does not exist, yet a need exists for a process for controlling the carbon content of injection molding steels during debinding.